(1) Field of the Invention
The present invention relates to a communication control method for sequentially allocating transmitting rights to a plurality of nodes commonly connected to a single transmission line.
(2) Description of the Related Art
FIG. 1 is a block diagram showing an example of the construction of a communication control system to which a conventional communication control method is applied. In FIG. 1, reference numerals 11 to 16 denote a plurality of nodes. In the figure, six nodes are illustrated as an example. Reference numeral 2 denote a transmission line connected to these nodes 11 to 16 to be commonly used by them.
FIG. 2 is a time chart explaining a conventional communication control method. In FIG. 2, reference numerals 31, 32, . . . , 36, 31, 32, . . . indicate periods of transmitting rights respectively allocated to the nodes 11 to 16. The transmitting rights are sequentially allocated to the nodes 11 to 16. Reference numerals 41 to 46 (in the figure, the reference numeral 45 does not appear) are frames when the nodes 11-16 do not have data to be transmitted, and the reference numerals 51-55 (in the figure, only the reference numerals 5S and 52 appear) are frames when the nodes 11-16 have data to be transmitted. In the illustrated example, during the period 35, the transmitting right is allocated to the node 15, and the node 15 has data to be transmitted, so that the frame 45, which is transmitted when there is no data to be transmitted, does not appear, but instead, the frame 55, which is transmitted when there is data to be transmitted, appears. Also, during the second period 32 from the left in the figure, the node 12 has the transmitting right and there is data to be transmitted so that the frame 42, which is transmitted when there is no data to be transmitted, does not appear, but instead, the frame 52, which is transmitted when there is data to be transmitted, appears. In the other periods 31, 32, 33, 34, 36, and 31, the frames 41, 42, 43, 44, 46 and 41, which are transmitted when there is no data to be transmitted, are transmitted from the corresponding nodes 11, 12, 13, 14, 16, and 11 respectively.
Next, the operation in the conventional communication control method will be described. In the communication control system shown in FIG. 1 in which devices used in factory automation (FA) and so on are connected through a time division multiplex communication network, a control method is conventionally employed in which, in order to ensure that each receiving time of each of the communication frames is within a previously designed limited time period, chances to transmit signals from the respective nodes 11 to 16 are given in the order of node numbers of the respective nodes. In such a communication control system, the nodes 11 to 16 respectively transmit data to be transmitted to the transmission line 2 by using the corresponding communication frames 51 to 56, when the nodes 11 to 16 obtain the corresponding transmitting rights during the corresponding periods 31 to 36 of the transmitting rights respectively allocated to the nodes 11 to 16. When there is no data to be transmitted when the transmitting right is allocated to the own node, it is necessary to inform all of the other nodes of the node number of the own node which occupies the corresponding period 31, . . . , or 36. To this end, the node generates a frame 41, . . . , or 46 in which data indicating that there is no data to be transmitted and the node number of the own node are included, and transmits the frame to the transmission line 2. Thereby, all of the nodes 11, . . . , and 16 recognizes each node number of each node which occupies each period 31, . . . , or 36.
As documents disclosing prior art relating to such a conventional communication control method, Japanese Patent Publication (Kokai) No. 59-139743, Japanese Patent Publication (Kokai) No. 62-281595, and so on are known.
In the above-described conventional communication control method, there is a problem as follows. Namely, even when the node 11, . . . , or 16 does not have data to be transmitted, the node must transmit the corresponding frame 41, . . . , or 46 including data indicating that there is no data and including the own node number during the period 31, . . . , or 36 allocated to the own node. Therefore, a problem arises in that it takes a long time to circulate the transmitting right from the node 11 having the minimum node number to the node 16 having the maximum node number 16.